Center for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium.
Chem Soc Rev. 2018 Feb 5;47(3):852-908. doi: 10.1039/c7cs00566k.
In pursuit of more sustainable and competitive biorefineries, the effective valorisation of lignin is key. An alluring opportunity is the exploitation of lignin as a resource for chemicals. Three technological biorefinery aspects will determine the realisation of a successful lignin-to-chemicals valorisation chain, namely (i) lignocellulose fractionation, (ii) lignin depolymerisation, and (iii) upgrading towards targeted chemicals. This review provides a summary and perspective of the extensive research that has been devoted to each of these three interconnected biorefinery aspects, ranging from industrially well-established techniques to the latest cutting edge innovations. To navigate the reader through the overwhelming collection of literature on each topic, distinct strategies/topics were delineated and summarised in comprehensive overview figures. Upon closer inspection, conceptual principles arise that rationalise the success of certain methodologies, and more importantly, can guide future research to further expand the portfolio of promising technologies. When targeting chemicals, a key objective during the fractionation and depolymerisation stage is to minimise lignin condensation (i.e. formation of resistive carbon-carbon linkages). During fractionation, this can be achieved by either (i) preserving the (native) lignin structure or (ii) by tolerating depolymerisation of the lignin polymer but preventing condensation through chemical quenching or physical removal of reactive intermediates. The latter strategy is also commonly applied in the lignin depolymerisation stage, while an alternative approach is to augment the relative rate of depolymerisation vs. condensation by enhancing the reactivity of the lignin structure towards depolymerisation. Finally, because depolymerised lignins often consist of a complex mixture of various compounds, upgrading of the raw product mixture through convergent transformations embodies a promising approach to decrease the complexity. This particular upgrading approach is termed funneling, and includes both chemocatalytic and biological strategies.
为了追求更可持续和更具竞争力的生物精炼厂，有效地利用木质素是关键。一个诱人的机会是利用木质素作为化学品的资源。三个技术生物精炼厂方面将决定成功实现木质素到化学品的增值链，即（i）木质纤维素的分馏，（ii）木质素的解聚，以及（iii）朝着目标化学品的升级。本文综述了广泛的研究，这些研究涉及到这三个相互关联的生物精炼厂方面中的每一个方面，从工业上成熟的技术到最新的前沿创新。为了帮助读者浏览每个主题浩如烟海的文献，我们划分并总结了不同的策略/主题，以综合概述图的形式呈现。仔细观察后，出现了一些概念原则，这些原则合理化了某些方法的成功，更重要的是，可以指导未来的研究，进一步扩展有前途的技术组合。当目标是化学品时，在分馏和解聚阶段的一个关键目标是最小化木质素的缩合（即形成有抗性的碳-碳键）。在分馏过程中，这可以通过（i）保留（天然）木质素结构，或（ii）通过容忍木质素聚合物的解聚但通过化学猝灭或通过物理去除反应中间体来防止缩合来实现。后一种策略也常用于木质素解聚阶段，而另一种方法是通过增强木质素结构对解聚的反应性来提高解聚与缩合的相对速率。最后，由于解聚的木质素通常由各种化合物的复杂混合物组成，因此通过收敛转化对原始产物混合物进行升级是一种很有前途的方法，可以降低复杂性。这种特殊的升级方法称为集束，包括化学催化和生物策略。
